1. Field of the Invention
The present invention relates to a method for optimally utilizing the efficiency of the gradient system a magnetic resonance of (MR) tomography apparatus and a device for implementing such a method.
2. Description of the Prior Art
Many pulse sequences for imaging with an MR tomography apparatus employ rapidly switched gradient fields. The efficiency of the gradient system cannot be fully utilized, since the rapidly switched magnetic fields, in the upper performance range, cause peripheral nerve stimulations (PNS) in the patient. The stimulation effects are caused by the electrical fields induced by the gradient fields in the body of the patient. The electrical field intensity grows with the amplitude and the frequency of the gradient field. The nervous system of the patient is activated when a physiology-dependent threshold value is exceeded. The patient perceives these stimulations as tingling or twitching in the body.
Clinical tests are currently carried out with new gradient systems, whereby the threshold values of a number of test persons are respectively determined. The limitations regarding the clinical operation of the device are determined from these individual thresholds on the basis of averaging. Although these fixed limitations generally prevent stimulations, a considerable range remains unused with respect to the majority of patients, since the fixed limitation must be set based on the particularly sensitive patient. Peripheral nerve stimulations still occur at individual patients even given this considerable limitation on the actual efficiency of the devices.
This limitation cannot be prevented either by a method for simulating electrical stimulations that are generated by gradient coils, as described in German OS 199 13 547. In such a system, the reference to the individual sensitivity of the individual patient is also missing, so that fixed stimulation values are assumed again, and the gradient field is no longer increased when they are reached.
An object of the invention is to provide a method and a device that is suitable for its implementation, so that it is possible to enable an optimal adjustment and complete utilization of the efficiency of the gradient system on an individual basis corresponding to the sensitivity of each patient.
This object is achieved in accordance with the invention in method and apparatus wherein the individually different sensitivity of each patient with respect to peripheral nerve stimulations is determined prior to the MR examination by applying a variable electrical field to the patient and wherein a test field is measured, the corresponding maximum magnetic field is determined by scaling from the test field, and the magnetic resonance apparatus is correspondingly adjusted so that the gradient system does not produce a magnetic field higher than the maximum magnetic field.
As a result of the inventive method of an individual, examination preceding determination of the sensitivity of the respective patient, an overall operation below the available efficiency of the devices is not necessary because the maximum magnetic field intensity that can be tolerated by the patient determined before the examination, so that the parameters are correspondingly optimized during the MR tomography sequence. This determination can be carried out while the patient being prepared for the MR examination, so that substantially not decrease in the patient throughput occurs. Moreover, there is no danger for the patient, since the generated test fields are locally extremely limited and the generated stimulations are extremely weak.
The variable electrical field can be generated by electro stimulations with a test electrode attached to the body of the patient. This presents a difficulty, however, is present, since the arising fields are extremely different from the fields induced by magnetic fields. To avoid this difficulty, in a further embodiment of the invention the variable electrical field is generated as an induction field by a test coil that is attached to the body of the patient. The same excitation mechanism is used for this type of prior testing of the sensitivity of the respective patient as for the MR examination, so that test measurements can be better compared with the later impairments as a result of the rapidly switching gradient fields of the MR tomography system. Preferably, the time function of the test fieldxe2x80x94regardless of whether it is a variable electrical field by means of electro-stimulation or a variable electrical field as induction field of a test coilxe2x80x94has stimulation-relevant characteristics that correspond to the time function of the respective MR gradient field in order to guarantee the same, and therefore closely comparable, conditions.
In addition to starting the electro-stimulation, which can be determined by interrogating the patient during the test, it has proven to be particularly expedient, as a result of the greater objectivity and the simpler applicability of the test results, to detect the peripheral nerve stimulation of the patient by the test field, that is chronologically offset vis-a-vis the excitation, using a measuring electrode. This measuring electrode can be arranged in the center of the test coil. Alternatively the never stimulation can be detected by the test electrode itself.
A device for implementing the inventive method has a measuring amplifier with an adjustable frequency for driving the test electrode or the test coil and for measuring and evaluating the signal of the measuring electrode and a conversion unit for scaling the test field with the MR gradient field.
Preferably, the conversion unit has a memory, in which the field calculation is stored for each gradient orientation and each patient position in order to enable a correlation with the test measurements and the utilized fields. As describe above the inventive arrangement and the inventive method allow the individual sensitivity of the patient to be measured prior to the MR examination and do not rely on calculated and estimated stimulation values, which are too small in many cases and too large in others.
The conversion unit can be integrated into the magnetic resonance apparatus, particularly as a sub-program with driving electronics and evaluation electronics, with the measuring amplifier preferably being wirelessly connected to the MR apparatus.